Project summary The choroid plexus is a highly vascularized, secretory tissue that protrudes into the brain ventricles, whose primary function is to produce cerebrospinal fluid (CSF). CSF provides nutritional and metabolic support for brain development and mediates efficient waste removal. Choroid plexus consists of an apical monolayer of ciliated epithelial cells that surround a stromal core of capillaries and connective tissues. Choroid plexus epithelial cells (CPECs) each forms one to two dozen cilia, which are microtubule-based organelles that project from the apical membrane. Emerging evidence has suggested the importance of cilia in choroid plexus development and function, as a connection has been established between a defective ciliogenesis in choroid plexus and the excessive CSF production in mice. Our preliminary studies identified a highly redundant miRNA family, miR-34/449 miRNAs, as the key regulators for CPEC ciliogenesis. The miR-34/449 family comprises six evolutionarily conserved, homologous miRNAs that are highly enriched in the ciliated CPECs. miR-34/449-deficient mice are characterized by reduced brain ventricle size, aberrant CPEC morphology and impaired brain development, suggesting a reduced CSF production due to defective choroid plexus functions. Interestingly, the earliest defect we can detect in miR-34/449-deficient choroid plexus is the aberrant ciliogenesis in CPECs, characterized by a significant increase of the cilium number per cell and a greater length of the axonemes. Given the functional importance of CPEC cilia in negatively regulating CSF production, we hypothesize that the miR-34/449 deficiency leads to excessive CPEC ciliogenesis, which represses CSF production and impairs brain development. Here, we propose to functionally characterize miR-34/449 miRNAs in CPEC ciliogenesis and CSF production, and to elucidate the underlying cellular and molecular mechanisms. Using genetic mouse models, choroid plexus in vitro culture, cell biology and molecular biology approaches, we will characterize the cellular and molecular defects in miR-34/449-TKO choroid plexus, and investigate the functional connection between CPEC cilia and choroid plexus function. We will also identify the key miR-34/449 targets that mediate CPEC ciliogenesis and CSF production. Our studies will not only reveal a highly robust regulatory mechanism for CPEC ciliogenesis, but also provide important insights into the functional importance of CPEC cilia in choroid plexus function.